Beaugrand Maïwenn, Arnold Alexandre A, Hénin Jérôme, Warschawski Dror E, Williamson Philip T F, Marcotte Isabelle
Department of Chemistry, Université du Québec à Montréal and Centre Québécois sur les Matériaux Fonctionnels , P.O. Box 8888, Downtown Station, Montreal, Canada H3C 3P8.
Langmuir. 2014 Jun 3;30(21):6162-70. doi: 10.1021/la5004353. Epub 2014 May 19.
Bicelles are model membranes generally made of long-chain dimyristoylphosphatidylcholine (DMPC) and short-chain dihexanoyl-PC (DHPC). They are extensively used in the study of membrane interactions and structure determination of membrane-associated peptides, since their composition and morphology mimic the widespread PC-rich natural eukaryotic membranes. At low DMPC/DHPC (q) molar ratios, fast-tumbling bicelles are formed in which the DMPC bilayer is stabilized by DHPC molecules in the high-curvature rim region. Experimental constraints imposed by techniques such as circular dichroism, dynamic light scattering, or microscopy may require the use of bicelles at high dilutions. Studies have shown that such conditions induce the formation of small aggregates and alter the lipid-to-detergent ratio of the bicelle assemblies. The objectives of this work were to determine the exact composition of those DMPC/DHPC isotropic bicelles and study the lipid miscibility. This was done using (31)P nuclear magnetic resonance (NMR) and exploring a wide range of lipid concentrations (2-400 mM) and q ratios (0.15-2). Our data demonstrate how dilution modifies the actual DMPC/DHPC molar ratio in the bicelles. Care must be taken for samples with a total lipid concentration ≤250 mM and especially at q ∼ 1.5-2, since moderate dilutions could lead to the formation of large and slow-tumbling lipid structures that could hinder the use of solution NMR methods, circular dichroism or dynamic light scattering studies. Our results, supported by infrared spectroscopy and molecular dynamics simulations, also show that phospholipids in bicelles are largely segregated only when q > 1. Boundaries are presented within which control of the bicelles' q ratio is possible. This work, thus, intends to guide the choice of q ratio and total phospholipid concentration when using isotropic bicelles.
双分子层囊泡是通常由长链二肉豆蔻酰磷脂酰胆碱（DMPC）和短链二己酰磷脂酰胆碱（DHPC）制成的模型膜。它们被广泛用于膜相互作用以及膜相关肽的结构测定研究，因为它们的组成和形态模仿了广泛存在的富含磷脂酰胆碱的天然真核细胞膜。在低DMPC/DHPC（q）摩尔比时，会形成快速翻滚的双分子层囊泡，其中DMPC双层在高曲率边缘区域由DHPC分子稳定。诸如圆二色性、动态光散射或显微镜等技术所施加的实验限制可能需要使用高稀释度的双分子层囊泡。研究表明，这种条件会诱导小聚集体的形成，并改变双分子层囊泡组装体的脂质与去污剂比例。这项工作的目的是确定那些DMPC/DHPC各向同性双分子层囊泡的确切组成，并研究脂质混溶性。这是通过使用（31）P核磁共振（NMR）并探索广泛的脂质浓度（2 - 400 mM）和q比例（0.15 - 2）来完成的。我们的数据表明稀释如何改变双分子层囊泡中实际的DMPC/DHPC摩尔比。对于总脂质浓度≤250 mM的样品必须小心，特别是在q ∼ 1.5 - 2时，因为适度稀释可能会导致形成大的、缓慢翻滚的脂质结构，这可能会阻碍溶液NMR方法、圆二色性或动态光散射研究的使用。我们的结果得到红外光谱和分子动力学模拟的支持，还表明只有当q > 1时，双分子层囊泡中的磷脂才会大量分离。给出了可以控制双分子层囊泡q比例的范围。因此，这项工作旨在指导使用各向同性双分子层囊泡时q比例和总磷脂浓度的选择。
